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Maria Campbell
Maria Campbell

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Clearing Terminal history in Linux and why it is important to do on occasion

#commandline #linux #terminalhistory #vim

Photo by Enes Bayraktar on pexels.com

This post was originally published on my personal blog mariadcampbell.com.

Table of Contents

What is the history command?

The history command in Linux (and macOS) stores a list of commands
that have been used in Terminal sessions, and it permits us to reuse
them instead of retyping them.

history is not actually a command

If I run which history in Terminal, nothing is stdout. This confirms
that history is not actually a command. history is a built-in keyword
of our shell (Another example is the cd command. It also is a builtin
keyword and not a command). And because history is written into the
particular shell we are using at any given time, there can be a
difference in its behavior across shells. But since I use bash in
Linux, I will be discussing bash-specific history behavior in my
particular Linux distro (Linux Mint).

history

If I run history in Terminal, I get back 428 lines, each line containing
a command I have run in Terminal. The following is a sneak peak at
history stdout:'

  1 ls
  2 cd Desktop
  3 ls
  4 cd cron-job-scripts/
  5 ls
  6 mailx
  7 sudo apt update
  8 sudo apt upgrade
  9 sudo apt install mailx
  ...
428 history
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If I wanted to run a specific command using its line number, I would
run the following:

!84
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Which would return the following:

cd desktop-backup/
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This is the command at line 84 in (Terminal) history. I could also
access and run this command by running the following:

!cd desktop-backup/
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Sometimes, as in this case, we get back something like the
following:

cd desktop-backup/ desktop-backup/
bash cd: too many arguments
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So I tried another one that does not result in any ambiguity:

!clear
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It runs the clear command, clearing the Terminal window. Using !
before the command itself prompts history to search for the last command
that matches the pattern I provided. In this case, it is clear.

But I digress. Let's get to the task at hand.

Removing history

There are several ways to remove some or all commands saved to history.

Deleting a command by line number

If I want to delete a command saved to history by line number, I would
run the following command:

history -d 423
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This removes the command at line 423. This is a great command to know
in case if you have mistyped a command, for example, and want to
remove it from history because it is essentially useless. But it can
also be used to remove specific commands containing
sensitive information. Why? What ifyou had to use history or
some other related tool or command in a presentation, and you wanted
to make sure that nothing sensitive would appear on the screen, and you
didn't want to completely remove your history. This would be one way
to do that.

history -c

The history -c command removes the contents of a specific Terminal
session's history. If I run history -c and then check to see if the
contents of my Terminal session's history was indeed completely
removed, I would run history again, and something like the following
would be returned:

1 history
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Manually removing the contents of .bash_history

history stores the commands run in Terminal in a file called
.bash_history, which is located in a user's home directory. If, after
running history -c, I want to double check if all my history has
actually been removed, I can run vim .bash_history to open up the file
in Vim. And something like the following would appear:

# The contents of .bash_history contained 426 lines. I am truncating it.
...
which diff
diff -s secret_message.txt decrypted_super_secret.txt
sha512sum secret_message.txt
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history -c only clears the history of a specific Terminal session, but
not the entire contents of .bash_history itself. Some articles state that
it completely removes all your history, but it does not. After running
history -c, your .bash_history still contains the history of the
commands you have run in Terminal.

If I wanted to manually remove the contents of .bash_history, I would
have to open the file and manually delete the contents of the file.
I could open the file using open .bash_history inside my home directory
where the file resides, Control + A the file contents, and then hit the
delete key and remove the file contents.

I could also run vim .bash_history and be taken into the Vim interface.
There, I would do the following:

# First I go into Vim normal mode by hitting the `esc` key
# Next, I enter command mode by hitting Shift key + : (colon)
# To Select All, I hit the Shift Key + % (5 key) after the colon (:)
# to delete everything, I then hit the D key after %
# To save my changes, I hit the esc key followed by Shift key + : (colon) and then the X key. This saves my changes and takes me out of Vim.
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If, after deleting the entire contents of .bash_history,
subsequent commands are not saved to .bash_history (it remains empty),
run the following command:

history > .bash_history
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Then open up .bash_history, and you should see your stored Terminal
history reappear. That is because you have redirected history as
stdout to the .bash_history file.

Next, to doubly make sure that everything is working as it should,
run a few commands in Terminal, and then re-open .bash_history to
see if those commands are being saved to .bash_history. When I
did this and re-opened .bash_history, I saw that new commands were not
being added to .bash_history. Even when I quit Terminal and started a
new session. So I had to do the following to do a history "reset":

# I ran this inside my home directory /home/maria where my .bash_history resides
rm .bash_history
touch .bash_history
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Once I re-created the .bash_history file, I saw that Terminal history
was saved to .bash_history, but commands from a Terminal session would
save to the .bash_history file when I would exit out of a Terminal
session and start a new one. But it worked just fine and as
expected!

Clearing .bash_history using the echo command

In order to remove to complete contents of the ~/.bash_history file, I
ran the following command:

# I did not have to add ~/ in front of .bash_history because I ran the command inside my home directory
echo > .bash_history
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And when I opened up .bash_history using open .bash_history, the file
was completely empty. This is the ONLY command that successfully removed
the contents of my .bash_history file!

bash history, privacy and security

Clearing one's bash history is not just about keeping things "clean".
it's also about safeguarding one's privacy and security. By clearing
our bash history on a regular basis, we can help protect
sensitive information that we may have used in a command, such as
passwords or other confidential data.

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